This invention relates to a process and apparatus for the oxygen delignification of fibrous materials, and more particularly to the medium consistency delignification of bleachable grade pulp and other fibrous materials using a series of substantially horizontal tubular reaction zones.
The control of pH during an oxygen delignification reaction has been recognized to provide beneficial effects such as improved pulp viscosity and strength as compared to simply adding the total charge of alkaline chemicals at the start of the reaction. For example, Grangaard et al., U.S. Pat. No. 2,926,114, teaches the oxygen delignification of wood chips by controlling the pH of the cooking liquor in the range of 7-9 during the major portion of the reaction. This pH control is achieved by using a buffer such as sodium bicarbonate in the liquor or by continuously adding alkaline chemicals such as sodium hydroxide or sodium carbonate throughout the reaction.
Samuelson, U.S. Pat. No. 3,769,152, teaches delignifying wood chips using an oxygen delignification process which involves the progressive addition of alkaline chemicals to maintain the pH of the cooking liquor in the range from about 9.5-13.
Kirk et al., "Low-Consistency Oxygen Delignification in a Pipeline Reactor," TAPPI, Vol. 61, No. 5 (May 1978) and Kirk et al., U.S. Pat. No. 4,198,266, teach that control of pH by the addition of alkaline chemicals in response to sensed pH changes along the length of a reactor produces improved pulp strength in an oxygen bleaching process on kraft pulp at 3% consistency compared to similar runs with no pH control.
Finally, Wallick, U.S. Pat. No. 4,248,662, describes an oxygen delignification system in which alkaline chemicals and recycled liquor are added along the length of a series of horizontal tubular reactors operating at from 3-8% consistency.
However, in all of the above described processes, the addition of alkaline chemicals during the oxygen delignification reaction presented no special problems with respect to uniform mixing of the added alkaline chemicals. In all of these processes of delignifying wood chips or pulp at low consistencies, free cooking liquor was available in addition to the liquor contained within the wood itself so that movement of the free liquor through the respective reactors served to distribute uniformly the added alkaline chemicals.
Processes which delignify pulp at medium (i.e., 8-20%) or high (i.e., 25-30%) consistencies do not have this free cooking liquor or only have insufficient quantities available to redistribute the added alkaline chemicals. Because the rate of oxygen delignification and the rate of alkaline chemical consumption increase dramatically as the concentration of alkaline chemicals increases, in areas of high alkaline chemical concentration the alkaline chemicals will be consumed rapidly before there is an opportunity for them to be redistributed. This may lead to pulp degradation in these areas. Additionally, high oxygen consumption in these areas may lead to oxygen starvation. All of these factors contribute to the production of a nonuniformly delignified pulp having less desirable strength and viscosity properties.
Attempts have been made to solve these problems in medium consistency operation by providing mixing equipment designed to mix uniformly the alkaline chemicals, oxygen, and pulp. For example, Kirk et al., U.S. Pat. No. 4,198,266, describes a "medium" consistency process which includes a plurality of mixing devices designed to generate high shear forces. Nasman et al., "Medium Consistency Oxygen Bleaching--An Alternative to the High Consistency Process," TAPPI, Volume 63, No. 4 (April 1980), describes a pilot plant operation which utilizes a steam mixer to mix steam and alkaline chemicals with the pulp and an oxygen mixer to disperse oxygen gas into the pulp prior to a vertical reactor. However, the use of such mixers is both complicated and expensive, especially when alkaline chemicals must be added at several locations during the delignification reaction. Moreover, the high shear forces created by such mixers may themselves cause degradation of the pulp.
Accordingly, the need still exists in the art for a relatively simple and economical process and apparatus providing uniform mixing and the controlled addition of alkaline chemicals to a medium consistency process for the oxygen delignification of pulp or other fibrous materials.